There is considerable interest today in large screen displays for computer, business and entertainment applications. Light valves fabricated using silicon chip technology combined with liquid crystal electro-optic properties are a major contender for this market. These devices require polarized light and careful design of the optical projection system.
FIG. 1 shows a typical optical system for a projection display utilizing silicon chip based liquid crystal light valves. The system includes an illumination system 100 and a projection optics system 102. The illumination system includes a lamp 104 which acts as the light source, a homogenizer (or integrator) 106 which evens out the illumination from the lamp 104 for uniformity of light on the display screen, a filter 108 which filters out ultra-violet (UV) and infrared (IR) light components, a mirror 110 which reflects the light toward the projection optics system 102 and eliminates residual IR light, and a series of lenses 112 which image the light for focus and magnification onto the light valves. The projection optics system 102 includes various mirrors 114 which isolate the red, green and blue components of the light, lenses 116 for imaging the light, polarizer sheets 118 for ensuring that the incident light is polarized in the correct direction, polarizing beam splitters (PBS) 120 together with reflective light valves 122 for controlling the light components, a color combiner 124 for redirecting light where necessary, and a projection lens 126 which focuses the light emitted from the color combiner 124 onto a distant screen (not shown).
FIG. 2 shows a reflective light valve 122 used in conventional combination with a PBS 120. The PBS 120 has the function of directing light of a particular polarization onto the light valve 122. The PBS 120 separates light according to its direction of polarization. The polarization (s polarization) of the incident light beam 200 is reflected by the dichroic surface 202 of the PBS 120 onto the light valve 122. The most common class of light valves in use today are based on polarization modulation, such as those using twisted nematic liquid crystal (TNLC). In reflective mode, these light valves neither absorb nor scatter the incident light. Application of a voltage to a light valve causes the polarization of the reflected (p polarization) beam to rotate and, therefore, to be transmitted through the PBS. Light reflected from regions where the applied voltage is zero remains in the original (s) polarization and, thus, is reflected back to the source. For this reason, light that remains in the original (s) polarization cannot contribute to the image. An alternative arrangement could be designed such that the incident light beam could be a p polarization beam operating in transmission through the PBS and the reflected beam could be an s polarization beam if rotated. In that case, the beams would be illustrated in the reverse of that of FIG. 2.
Thus, in FIG. 2, where the light valve 122 is normally in the dark state and no voltage is applied thereto, the s polarization light beam 200 is reflected back toward the source.
When a voltage is applied to the light valve 122, the polarization direction of the light beam is rotated into a p polarization light beam 204 which is transmitted through the PBS 120 toward the screen (not shown). Thus, in a projection display that uses reflective light valves, s polarization and p polarization beams must both propagate in the space between the PBS and the light valve.
With normally white light valves, the polarization is rotated when the applied voltage is zero. When a voltage is applied to the light valve, the beam is reflected with some polarization. Only near maximum voltage is the beam reflected without rotation of polarization. In any case, whether the light valve is normally white or normally black, light that remains in the original polarization will be returned to the source. That is, light that is unmodified by the light valve is, thus, prevented by the PBS from reaching the screen.
Typically, the illumination produces a light beam where individual rays lie within a range of angles about a principal direction (shown in FIGS.). For rays that are at an angle from the principal ray shown in FIG. 2, the dark state includes a particular orientation of the rays, as well as their polarization. Regions of the light valve that display black patterns will reflect light in the dark state, while regions set at maximum brightness will produce a reflected beam in the rotated bright state polarization. The optical system must contain an element like the PBS to separate the bright state from the dark state light. The PBS passes bright state light through the projection lens to form bright areas of the screen image. Black areas of the image contain no light since dark state light is removed by the PBS. In some cases, like that shown in FIGS. 1 and 2, the hypotenuse coating of the PBS is the only tilted coating to which both polarizations in the reflected beam are directed. By the time the reflected beam reaches other tilted coatings, the PBS has already removed the dark state light. In other cases, such as the example shown in FIG. 7, there may be several tilted coatings that see both bright and dark state polarizations. For good contrast, it is necessary that dark-state polarized rays remain entirely in the dark state as they propagate through all tilted coatings. If small amounts of light from dark-switched regions are converted to bright state, the mispolarized light will be passed by the PBS, producing background intensity in nominally black regions of the image. While supplementary sheet polarizers are a known method for filtering out light that gets converted to the wrong polarization, these devices cannot be employed where both polarization states must be allowed to propagate.
The ratio of the amount of light reaching the screen when the light valve is activated to the small unwanted leakage of light through the polarizing beam splitter when the screen should be dark, largely determines the display image contrast that the projector may achieve. This is an important quality factor for the user.
It is understood, in the prior art, that light leakage is related to geometric effects associated with rays passing through the tilted coatings in the optical system at an angle to the principal axis. See A. E. Rosenbluth et al., "Contrast Losses in Projection Displays from Depolarization by Tilted Beam Splitter Coatings," in Proceedings of 1997 International Display Research Conference (Toronto: Society for Information Display, 1997), p. 226 ("Contrast Losses"); A. E. Rosenbluth et al., "Contrast Properties of Reflective Liquid Crystal Lightvalves in Projection Displays," IBM Journal of Research and Development 42, no. 3/4 (1998), pp. 359-386 ("Contrast Properties") which are hereby incorporated by reference. If the ray angle has a skew component relative to the tilt in the surface, its plane of incidence will be different from that of the principal ray due to the compound angle involved. In the case of the principal ray, the electric field will be entirely parallel or entirely perpendicular to the plane of incidence, but this will not be true of skew rays. For this reason, tilted coatings will usually change the polarization of rays with a nonzero skew component, causing polarization cross-talk. Typically, the rays illuminating a light valve must occupy a range of angles (.about.10.degree.) so that a beam derived from a thermal source (e.g., arc lamp) contain enough energy to provide competitive brightness. The necessary angular range can be decreased if beam diameter is increased, but this runs counter to the goals of compactness and cost minimization with use of small components. For beams subtending angles .about.10.degree., the compound angle depolarization of skew rays is often the dominant factor limiting full-screen contrast.
A method is known in the prior art for correcting this compound angle effect in the following special case: When the polarizing coating on the PBS hypotenuse is the only tilted coating that both polarizations pass through, as in FIGS. 1 and 2, and when the light valve is mirror-like in the dark state, it is known that compound angle depolarization may be corrected by disposing a quarter wave plate 300 between the polarizing beam splitter 120 and the light valve 122, as shown in FIG. 3. Two passes through the quarter wave plate 300 eliminates angular geometrical depolarization. This is further shown in European Patent No. 389,240 entitled "Polarizing beamsplitter apparatus and lightvalve image projection system," issued 1990 to Y. Miyatake and assigned to Matsushita Electric Industrial Co. ("Miyatake").
The conventional quarter wave plate method has several deficiencies, however. First, as noted, it is only applicable in the case of PBS cube systems like that in FIGS. 1 and 2 where no other tilted coatings are present between the light valve and the PBS. Thus, it is not effective in the case of more complex optical systems like that shown in FIG. 7. Second, many light valves are not precisely mirror-like in the dark state. That is, they have optical properties that vary with wavelength so that the quarter wave plate is not effective for all wavelengths. One difference is that the light valve may itself exhibit imperfect contrast, i.e. it may introduce a small amount of depolarization in the incident light. In a high quality light valve, this depolarization (denoted B.sub.LV) is small. For example, curve A 401 in FIG. 4 shows the black state reflectivity (reflectivity intensity toward the screen in the dark state) of a twisted nematic light valve designed to operate in the green channel of a projector, as discussed in Contrast Losses and in K. H. Yang and M. Lu, "Nematic LC Modes and LC Phase Gratings for Reflective Spatial Light Modulators," IBM Journal of Research and Development 42, No. 3/4 (1998), p. 401-410. An incoherent contribution to background of 1 part in 800 is assumed for purposes of illustration. Except for this incoherent scatter, the light valve is mirror-like at the central green wavelength of 550 nm, and the black state intensity is 1/800th that of full intensity green. At other green wavelengths, the light valve is not mirror-like and B.sub.LV (curve A) is larger than 1/800. However, integrated over the green band between about 510 nm and 590 nm, the light valve black state contribution is reasonably small in comparison with a typical system contrast requirement, which might, for example, be 300:1. The contrast of the light valve is, thus, reasonably good, but unfortunately, as will be discussed further hereinbelow, there is an interaction between a reflective polarizing light valve and projection optical systems which almost always causes the contrast of the complete projector to be poorer than that of the light valve alone.
Curve A in FIG. 4 assumes that the TNLC light valve is illuminated with light that is linear polarized in an optimum direction (dark state polarized). Because of depolarization introduced by the optical system, the illuminating light may not actually achieve such a pure polarization state. This depolarization does not involve the light valve, but if the light valve is not mirror-like, it may contribute a further depolarization, and also a retardance, i.e. a differential phase shift between the two illuminating polarization components, producing effects not seen in curve A of FIG. 4. The end result is that, in a projector, the contrast is almost always poorer than curve A, i.e. the residual black state intensity of the system is almost always larger than that of the light valve alone.
As noted above, the PBS introduces a depolarization in rays that have a skew component of incidence at the tilted hypotenuse coating. If the light valve in FIG. 2 is replaced with an analyzing polarizer that is crossed to the pass direction of the PBS (pass polarization is S, analyzer is set to P), the depolarization of the PBS can be measured in single pass. For a uniform cone of rays subtending an angle NA, the transmitted single-pass intensity is given by (NA/2n).sup.2, where n is the refractive index of the PBS substrates. Customarily, NA denotes the sine of the cone angle, but for cone angles .about.10.degree., the distinction is numerically small. Using NA=0.167 and n=1.7, the single-pass black state contribution of the optics, denoted B.sub.Optics, is thus about 1/400(where unity represents full bright state intensity). The single pass PBS depolarization, B.sub.Optics, thus does not exceed the 300:1 system contrast requirement assumed here for purposes of illustration.
However, a much larger background occurs in the projection system, where the PBS is traversed in double pass by the light, preceding and following reflection from the light valve. Black state reflectivity in this case (denoted B.sub.System) is plotted as curve B 403 in FIG. 4 for the TNLC light valve whose black state is curve A 401. Black state light leakage in the green band is seen to be about 1 part in 100. While this poor system performance can be considerably improved by placing a quarter wave plate 300 above the light valve, as per FIG. 3, reducing the black state reflectivity to the level plotted as curve C 405 in FIG. 4, the contrast is not as high as is achieved by the light valve alone (curve A 401). Moreover, if the NA of the system is increased for better light collection, the excess black state intensity in curve C 405 increases quadratically. It is desirable that the dark state intensity be as low as possible for good contrast, but at the same time, the cone of rays should be as large as possible to transfer as much light as possible through the system. Subject to constraints of proper primary chromaticity, it is also desirable that a broad band of wavelengths be included in each color channel, since this also increases brightness (with a thermal source). The curve C 405 intensity rises as wavelength deviates from the central wavelength. Because each light valve must work with a range of wavelengths within a color band, not just at a single wavelength, interaction between the light valve and optical system results in appreciable black state light leakage.
Therefore, there is a need for a system and method for realizing the high contrast obtainable with reflective polarization-modulating light valves when the light valves are used in a projection system.